1. Field of the Invention
The invention relates to a process and apparatus for optically checking perforations in hollow articles, and is particularly concerned with the checking of micro-perforations in aircraft engine parts such as combustion chamber walls, multi-perforated jackets of after-burner passages, and hollow turbine blades.
In such parts it is essential to be able to check, or even measure, the "permeability" of very small diameter holes. This permeability represents the capacity of the holes (perforations) to allow an aeration or cooling air flow through the wall in which they are drilled, and compliance with a minimum permeability is generally vital to the life of the part concerned.
Thus, in one example of cooled blades for the first turbine stage of a supersonic turbojet engine, in which the internal cavity of the blade is supplied with pressurized air through its root, each blade has one row of 53 perforations along its leading edge and two rows of 80 and 19 perforations along the trailing edge, all of the perforations having an average diameter of from 300 to 500 microns.
2. Summary of the prior art
The present method for checking the drilling quality of these holes consists of performing manually two operations. Firstly a gauge rod is inserted into each hole to check whether the hole is of a minimum diameter equal to that of the rod and whether the hole opens out into the internal cavity of the blade, and secondly the holes are counted to check that the intended number of holes have been provided.
In addition to the fact that this method is lengthy and tedious, the risks of error are great when counting the holes, and when checking the drilling by means of gauge rods, some holes may be omitted if the operator's attention is distracted. Moreover, these checks do not indicate whether the holes have been drilled in the right place, or whether the drilling accuracy (non-circular, etc.) is correct.